It has long been known that an inorganic coating can be applied to a glass surface by contacting a hot glass surface with a thermally-decomposable metal compound, whereby the compound, when brought in contact with the hot glass, decomposes to form what is believed to be a metal oxide layer on the surface of the hot glass. Processes of this type were disclosed by Lyle in U.S. Pat. No. 2,375,482 and U.S. Pat. No. 3,353,514.
More recently, processes of this type have been adapted for use in protective coatings for glass containers, particularly beverage bottles and other similar containers. In these processes glass containers, such as glass bottles, while still hot from the bottle-forming equipment and before passage through the annealing lehr, are treated with a thermally-decomposable metal compound, usually stannic chloride, under conditions such that a thin coating is formed on the container surface, which serves to anchor a lubricious organic polymer or wax coating applied to the container surface after they exit from the lehr. This combination of metal oxide "hot end" coating and organic "cold end" coating has been found useful in improving the scratch resistance and lubricity of glass containers. Processes of this type are disclosed, e.g., by Gatchet et al in U.S. Pat. No. 3,516,811.
Although this combined coating has been found useful, current methods of applying hot end coatings have several drawbacks. One of the major methods in commercial use involves applying vapors of the thermally decomposable compound to the hot glass surface. In such a method, it was believed that the metal compound, especially is stannic or titanic chloride is used, must be anhydrous and that dry conditions must be employed throughout. If the metal compound vapors are contacted with moisture, a reaction occurs which leads to the formation of a white solid which can plug vapor lines. In addition, if a halide is employed, such a reaction can lead to the formation of hydrogen halide vapors, e.g., vapors of hydrogen chloride, which are highly corrosive.
Furthermore, in the known procedures it is difficult to ensure formation of a uniform hot end coating because, when the metal compound fumes react with moisture in the atmosphere before contacting the glass surface, non-uniform coating thicknesses and poor bottle-to-bottle reproducibility are obtained. Moreover, the loss of metal halide through such a reaction seriously reduces the efficiency of the use of the expensive metal compound reagent.
Furthermore, in the known processes, only a small portion of the metal compound is actually converted into the inorganic hot end coating; most of the compound passes across the conveyor. Consequently, considerable effort has been made to devise equipment to recapture and recycle the metal compound, as is reflected, e.g., in U.S. Pat. No. 3,688,737 to Augustsson et al. Other efforts to improve coating efficiencies included application of the metal compound in the form of a liquid spray. One such method, employing an aqueous solution of a tin chloride hydrate, is described by Southwick et al in U.S. Pat. No. 3,819,346. Despite this effort, the commercially employed hot end coating procedures still remain relatively inefficient. Recent increases in the cost of the coating agents, especially in the cost of stannic chloride, have given even further impetus to the development of more efficient coating techniques.